1. Field of the Invention
This invention relates to storage media and more particularly relates to accessing discontinuous media tracks on a storage medium.
2. Description of the Related Art
Traditionally, the storage capacity and the areal density of magnetic storage media, such as disks used within disk drives, have been limited by certain restraints such as material characteristics, manufacturing processes, metrological limitations, mechanical capabilities, and the like. For example, conventional multigrain magnetic media is generally created by covering a flat substrate with a thin layer of magnetic alloy that forms random clusters of magnetically charged grains on the substrate surface.
The conventional process of creating media storage has been limited by several physical constraints, including a natural occurrence called the superparamagnetic effect, i.e. the fluctuation of magnetization due to thermal agitation. The superparamagnetic effect influences a storage medium when bit cells of magnetically charged grain clusters, are defined by grains so small that magnetization becomes unstable. In such circumstances, fluctuation of magnetization can cause erasure of data. Consequently, the areal density of thermally stable storage media has typically been restricted to around 150 Gbit/in2 with conventional multigrain magnetic media.
Recently, however, patterned media comprising an ordered array of highly uniform islands has been developed as an alternative to circumvent some of the limitations traditionally associated with magnetic media. Independent magnetic islands corresponding to bits or track segments that are thermally stable can be formed on the surface of the storage media. Each island of the patterned media may be capable of storing one or more bits. As a result of the magnetic isolation provided by the islands, storage media can be created with greater areal densities and storage capacities than possible with conventional multigrain magnetic media technology.
Patterned media can be formed by a variety of methods known to those skilled in the art. One proposed method to create patterned media is nanoimprint lithography or nanoimprint replication. In this method, a stamper, or a master template, may be formed having a nm-scale pattern. The stamper may subsequently be used to stamp, or create, formed patterns on the surface of a substrate at a relatively low cost compared to other methods for patterning. The formed patterns may be covered with a magnetic layer to form independent magnetic domains.
One method suitable for creating a patterned stamper, either directly or indirectly, uses e-beam lithography. E-beam lithography, in certain embodiments, can be used to create islands having dimensions around the scale of about 25 nm or less, corresponding to areal densities of about 300 Gbit/in2 or greater. These high resolution patterns are beyond the dimensions achievable using optical lithography, a technique commonly used in the electronics industry to make integrated circuits. E-beam exposure tools may be similar to those used to create masks for optical lithography; however, generating patterns for patterned media generally requires a higher resolution e-beam exposure system than is needed for conventional e-beam mask generation.
Patterned media, which overcomes some of the traditional challenges associated with magnetic media, can beneficially provide storage media with greater areal densities and storage capacities. However, the mass production of patterned media presents some challenges that have hindered patterned media from becoming readily available in the market.
First, because predefined patterns are typically formed into the substrate of a storage medium or disk, tracks and bit alignment is fixed during fabrication instead of during a servowriting process as is commonly done with conventional magnetic media. Consequently, storage devices and read/write mechanisms must be able to follow preformed tracks and compensate for errors, such as centering errors, shape irregularities, or the like. Ideally, a servo system would able to function efficiently despite misaligned features within the patterned media. However, due to the high flying speeds and small features associated with reading and writing data, currently available servo systems are able to adjust to only minor variations within the patterned media.
Second, creating patterns for master templates by writing high resolution patterns using e-beam lithography can be prohibitively time-consuming. Typically, e-beam lithography tools operating at their best resolution are using the smallest beam diameter and a very limited current. Depending on the detail of the pattern, the beam current available, the sensitivity of a selected resist, and the time associated with mechanical motion of the stage, the total write time required to create a pattern for a whole disk surface may take months to years to complete, depending on the size of the pattern. Larger patterns require greater amounts of time for completion. Drawn out write times may be unacceptable, considering the associated cost and the delicacy of the e-beam writing system. In certain instances, the e-beam may fail long before the completion of the e-beam writing process.
A contemplated solution to reduce the size of e-beam written patterns is to create a small section of a pattern using e-beam lithography, and then to replicate the section using other methods. Media patterns tend to be periodic, and repeated patterned sections do not pose serious problems. However, forming multiple sections on the surface of a storage medium presents several physical limitations that affect the continuity of individual data tracks around the disk. Due to the scale of the pattern features, perfect alignment of track segments from individual sections is nearly impossible and would require extremely sensitive manufacturing environments that for the foreseeable future appears to be too costly for practical use. Furthermore, misaligned media tracks from a more cost-effective production environment may disrupt a servo system when transitioning between patterned sections.
From the foregoing discussion, it should be apparent that a need exists for a method, apparatus, and system for accessing discontinuous media tracks created in a cost-effective production environment. Beneficially, such an apparatus, system, and method would enable storage media to maximize the areal density and storage capacity of a given storage area and would enable economical and efficient implementation of high resolution patterned media.